With known ellipsometers, a sample is exposed to radiation by a linear-polarized laser beam, for which purpose a laser with a polarizer and a compensator is arranged. The laser beam impinges obliquely on the sample, is reflected on its surface and is directed in the form of an elliptically polarized beam to a photo detector via a rotating analyzer. As a rule, the output signals of the photo detector are supplied to a computer for evaluation of the measuring signals. In ellipsometry, the change of the polarization state of the reflected light is measured. The reflection ratio between parallel and vertically polarized light is a function of the layer thickness, among others (DE-OS 39 26 184, DE-OS 41 08 329, the disclosure of which is expressly incorporated herein by reference).
The advantage of ellipsometry as compared to, for example, photometry consists in that the independent measurement of two parameters, for example thickness and refractive index, is possible in one measuring operation. Furthermore, a rapid measurement of high accuracy is possible.
The disadvantage of simple ellipsometers operating with one wavelength lies in that no spectral information is obtained and that measurement of the layer thickness is only possible within one ellipsometric period. If there is no statement regarding the period, it is also not possible to make a statement regarding the absolute layer thickness.
A further disadvantage lies in that high measuring accuracy depends on the film thickness if the refractive index is unknown. The interrelationship is shown in FIG. 1. The ellipsometric angles psi and delta for different refractive indices are shown there. In the area of the focus, which is located at a delta value of 180.degree., i.e. when measuring layers of a thickness of approximately 220 to 340 nanometers, an independent measurement of the layer thickness and of the refractive index is not possible.
Spectral photometers (DE-OS 39 26 184) are also known for measuring the thickness of transparent layers. With these, the layer to be tested is illuminated with white light, i.e. with light having a sufficiently large range of wavelengths. If such radiation is reflected by a transparent layer, the portions reflected at the front and the rear boundary surface travel over different distances.
When they are superimposed, interferences are created, i.e. the reflected radiation is either increased or decreased or cancelled as a function of the optical layer thickness and wavelength. The reflectivity is a function of the product of refractive index times layer thickness, so that it is possible to detect the layer thickness if the refractive index is known.
Measuring with the spectral photometer has the disadvantage that the simultaneous determination of the layer thickness as well as of the refractive index is not possible in one measurement because of the spectral dependence on the refractive index.
The mentioned disadvantages can be avoided by using a spectro-ellipsometer. With this, the measurements are performed in a continuously variable spectrum.
However, the disadvantage of the spectro-ellipsometer lies in that its construction is very elaborate and thus expensive.
It is the object of the invention to determine, in the course of ellipsometric measurements of transparent films on reflecting substrates at one wavelength or with a few discrete wavelength, at least two characteristic values of the film, such as layer thickness and refractive index and the dispersion, with great precision, even with layer thicknesses wherein at least one of the characteristic values at one wavelength or a few discrete wavelengths could not or could only be inaccurately detected up to now, and in addition to determine the spectral dependence of the characteristic values.